The present invention relates to radio communications. More particularly and not by way of limitation, the present invention is directed to a system and method for communicating with aircraft through cellular base station towers.
For air traffic control communications, civil aircraft in flight communicate with ground stations utilizing narrowband channels in the 116-174 MHz region of the radio spectrum. Aircraft are also assigned a 2-MHz segment of the spectrum in each of an uplink band and a downlink band in the 800-900 MHz region for extending access to terrestrial communications networks to passengers in flight. The latter has been used to provide telephone services via seat-back phones to the passenger cabin.
In a related area, cellular communication networks in the U.S. are assigned a much wider segment of the spectrum for communications with mobile stations. For example, cellular communication networks operating in the 1900-MHz band are assigned a 60-MHz segment in both the uplink and downlink directions. Modern cellular communication networks are upgrading to higher data rates to provide Internet communications and other so-called multi-media communications to mobile subscribers. The Internet is a medium that has become as ubiquitous as the telephone and favors use of much higher bandwidths than telephony. Such broadband services are not available for communications with aircraft because they cannot be provided satisfactorily within the 2-MHz segment currently assigned to ground-to-air communications.
FIG. 1A is a top view of a typical antenna radiation pattern illustrating a horizontal radiation pattern for an existing cellular base station tower. In the illustrated configuration, a centrally located sectorized antenna transmits and receives RF signals in Sector-A, Sector-B, and Sector-C of a cell defining the service area of the base station. In the horizontal plane, each of three separate antenna arrays covers an associated 120° sector of azimuth. The horizontal beam shape is usually chosen to be about −12 dB from the peak of the main lobe at the +/−60° sector edge, since only half the maximum range is needed in that direction. The −3 dB beamwidth is of the order of +/−30°, which is the same as or similar to what would have been used to cover 60° sectors.
FIG. 1B is a side view of a typical antenna radiation pattern illustrating a vertical radiation pattern for Sector-A of the existing cellular base station tower of FIG. 1A. Although not illustrated, the vertical radiation patterns in the other sectors are similarly positioned in the vertical plane. The pattern in each of the antenna sectors is formed as a horizontally oriented lobe. In the vertical plane, the beamwidth is typically narrower because there is rarely any requirement to cover stations other than at ground level. In some cells, the antenna pattern may be tilted down a small amount (for example 5°) depending on the tower height, cell size, and terrain. The typical directive gain of a cellular base station antenna is 18 dBI, which may comprise a factor of 6 (i.e., 8 dB) directivity in the horizontal plane and a factor of 10 (i.e., 10 dB) in the vertical plane. The 10 dB gain in the vertical plane may include a 4 dB vertical directivity gain for each antenna element and a 6 dB array gain due to co-phasing four such elements.
The vertical radiation pattern for existing cellular base station towers is thus optimized for communicating with mobile phones on the ground or in buildings. The radiation pattern provides insufficient gain at the higher elevation angles needed to communicate with aircraft in flight.
It is desirable to provide wideband communications to aircraft in flight to provide a variety of multi-media services. It would be advantageous to have an antenna arrangement, system, and method for providing wideband communications services to aircraft without demanding more of the radio spectrum and without interfering with ground-based subscribers. The present invention provides such an arrangement, system, and method.